Sema: Validate extension if needed in isExtensionApplied()

We're looking at the generic signature of the extension, so it must
have been validated at this point.

I don't have a test case for this and maybe it never came up, but
nothing forces it to be validated here so let's make it more robust
by doing it explicitly.

Author: slavapestov

lib/Sema/CSGen.cpp

@@ -3704,6 +3704,7 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
     return true;
 
   TypeChecker *TC = &createTypeChecker(DC.getASTContext());
+  TC->validateExtension(const_cast<ExtensionDecl *>(ED));
 
   ConstraintSystemOptions Options;
   ConstraintSystem CS(*TC, &DC, Options);